1. Field of the Invention
The present invention relates to an image signal recording apparatus for recording an image signal on a recording medium.
2. Description of the Related Art
It is known that various television systems such as NTSC systems and PAL systems have conventionally been used in the field of still image signal recording/reproducing systems of the type which are arranged to record a still image signal on a recording medium and to reproduce a still image signal recorded on the recording medium. In general, the still image signal recording/reproducing systems utilize a method including the steps of frequency-modulating image signals of a format according to such a television system and magnetically recording the modulated image signals on small magnetic disks called video floppy disks. The resolution represented by the image signal recorded by the above method is substantially equal to that realized by the current television systems. However, if an image reproduced by such a still image recording/reproducing system is printed out by means of a printer or the like, image quality, particularly resolution, is low as compared to the resolution of a typical silver salt photograph.
In recent years, new television systems, such as a high-definition television (HDTV) system, have been investigated and proposed. The HDTV system realizes the vertical resolution of approximately 1,000 scanning lines per picture and has a signal band which provides horizontal resolution corresponding to the vertical resolution. For this reason, as a still image signal recording/reproducing system corresponding to the HDTV system, it is desired to provide a system capable of recording and reproducing still image signals which assure image quality of the order of 1,000.times.1,000 pixels.
In such circumstances, a recording/reproduction method which makes it possible to record still image signals with image quality as high as 1,000.times.1,000 pixels while retaining compatibility with the recording format used in a conventional type of still image signal recording/reproducing system, has previously been proposed in U.S. patent applications Ser. No. 334,305 filed on Apr. 27, 1989, Ser. No. 344,202 filed on Apr. 27, 1989, Ser. No. 345,411 filed on Apr. 28, 1989, Ser. No. 450,393 filed on Dec. 14, 1989, Ser. No. 457,275 filed on Dec. 27, 1898, Ser. No. 460,308 filed on Jan. 3, 1990 and Ser. No. 460,306 filed on Jan. 3, 1990. The above method and the conventional method will be hereinafter referred to as a "CHSV (Compatible High-Definition Still Video) method" and a "SV method", respectively.
The operation of recording a luminance signal according to the CHSV method will be explained below. FIG. 1 partly shows the sampling positions of each luminance signal Y on a picture, which luminance signals Y are recorded on a video floppy disk by the CHSV method. The luminance signals Y (or Y signals) are sub-sampled and recorded in such a manner that each line of sample points is alternately offset in relation to the next adjacent line of sample points. For reproduction, the alternating sample points are converted into a lattice-like matrix of sample points by means of interpolation. The number of sample points is 600 (=1,200/2) per line and 500 (=1,000/2) per column, and information representing all the sample points in one picture is recorded on a total of four tracks. Referring to FIG. 1, for example, information representing the sample points contained in each line A.sub.1, A.sub.2, . . . is recorded on one particular track of the video floppy disk, information representing the sample points contained in each line B.sub.1, B.sub.2, . . . is recorded on another track of the video floppy disk, and so on. Recording on each track is performed in accordance with the recording format of the SV method, and the basebands of the Y signal and a chrominance signal C (or C signal) are approximately 7 MHz and approximately 1 MHz, respectively. In the sampling method shown in FIG. 1, the number of Y-signal sample points is, as described above, approximately 600 per line, and these sample points are inserted in the horizontal effective picture period (approximately 53 .mu.s) of an NTSC television signal. Accordingly, the corresponding sampling period is approximately 5.7 MHz.
In the CHSV method, information representing the sample points for one picture is recorded on four tracks, and, when the application of the CHSV method to the camera-integrated type is taken into account, the time required for recording needs to be minimized to prevent image quality from being deteriorated by an increase in dark current in an image sensor. Accordingly, in the CHSV method, information is normally recorded on two or four tracks at the same time.
FIGS. 2(a) and 2(b) show the patterns of track recordings on a video floppy disk according to the CHSV method, respectively. In each of FIGS. 2 (a) and 2(b), the right-hand and left-hand ends correspond to the inner and outer circumferences of the video floppy disk. FIG. 2(a) shows the case of a two-channel head, and FIG. 2(b) shows the case of a four-channel head. Of course, the four-channel head can be used to effect recording in accordance with the recording pattern of FIG. 2(a).
Referring to the case shown in FIG. 2(a), information representing the sample points of the Y signals in lines A.sub.i (i=integer) and lines B.sub.i are initially recorded on first and second tracks at the same time. Then, the head, if of the two-channel type, is shifted to third and fourth tracks, and records information representing the sample points of the Y signals in lines D.sub.i and lines C.sub.i on the respective tracks at the same time. In the case of a four-channel head, the head is not shifted to effect similar recording. To assure compatibility with the SV method, the order of the lines D.sub.i and C.sub.i is reversed as illustrated. As is well known, the above two-track simultaneous recording generally has the problem that crosstalk signals occur between by two heads during recording. However, the above-described recording method can be used to solve such a problem by effecting well-known "horizontal (H) alignment" between the two heads during simultaneous recording.
In the case of the four-channel head shown in FIG. 2(b), information representing the Y-signal sample points in the lines A.sub.i and B.sub.i is recorded on the first and third tracks, respectively, at the same time, and information representing the Y-signal sample points in the lines C.sub.i and D.sub.i is recorded on the second and fourth tracks, respectively, at the same time.
In the track recording pattern shown in FIG. 2(a), the second and third tracks can be used to effect frame-mode reproduction based on the SV method. In the track recording pattern shown in FIG. 2(b), the first and second tracks or the third and fourth tracks can be used to effect frame-mode reproduction based on the SV method.
The operation of recording a chrominance signal C by the CHSV method will now be explained below. FIGS. 3(a), 3(b) and 3(c) show the relation between sample points on a picture. The sample points of FIGS. 3(a), 3(b) and 3(c) are respectively derived from Y signals, C.sub.R (=R-Y) signals and C.sub.B (=B-Y) signals, all of which are recorded on a video floppy disk. Of the sample points, sample points corresponding to recorded information are indicated by ".largecircle." and sample points corresponding to information to be interpolated are indicated by ".multidot.". In the recording format according to the SV system, a recording band for the C signal is approximately one sixth of that for the Y signal, and the C signal is recorded in a line-sequential manner. Accordingly, the positions of the sample points of the color-difference signals C.sub.R and C.sub.B are as shown in FIGS. 3(b) and 3(c), respectively. Throughout FIGS. 3(a), 3(b) and 3(c) each of which shows the lines A.sub.i and B.sub.i as well as lines C.sub.i and D.sub.i, the Y signals, the C.sub.R signals or the C.sub.B signals recorded on the same track of the video floppy disk are indicated by the identical symbol A.sub.i, B.sub.i, C.sub.i or D.sub.i. In FIGS. 3(a), 3(b) and 3(c), although there is a positional discrepancy between the Y-signal lines and the corresponding C-signal lines, this discrepancy is needed to assure compatibility with the SV system (for example, and intra-field color-difference line-sequential operation).
As is apparent from the foregoing, the CHSV method is a recording method based on the assumption of retaining compatibility with the recording format of the SV system. However, in the recording format of the current SV system, an image signal and an ID code are recorded together, and the ID code includes identification information for identifying frame recording or field recording. An important consideration in the CHSV method is, therefore, how to handle such frame recording/field recording identification information.